This invention relates generally to mass spectrometers and to collision cells used therein, and more particularly the invention relates to a lens-free ion collision cell.
Mass spectrometers are well known scientific instruments for analyzing chemical structures. A mass spectrometer includes an ion source, an ion filter, and an ion detector. Analyte is introduced into the ion source which ionizes the material. Ions are then selected by the ion filter and passed to the ion detector. The ion filter selects ions having a particular m/e ration which may be varied to analyze the gas. Examples of mass spectrometers are described in U.S. Pat. Nos. 5,559,327; 5,389,785; 5,298,745; 4,949,047; 4,885,470; 4,158,771; and 3,757,115.
These and other known mass spectrometers employ filters having linear hyperbolic quadrapoles. FIG. 1 is a schematic perspective view of the ion filters described in U.S. Pat. No. 5,389,785 to Urs Steiner et al. The filter includes four linear rod electrodes 3, each rod electrode having a hyperbolic curved pole surface 5 and two abutment surfaces 7 so that the rods can be assembled with opposing pairs of hyperbolic surfaces. Rods with opposing hyperbolic surfaces are electrically connected, and both RF and DC voltages are impressed on the rods with the RF voltages on adjacent poles being out of phase and the DC voltages on adjacent poles being offset.
FIG. 2 is a plan view of a mass spectrophotometer employing hyperbolic quadrapoles as disclosed in U.S. Pat. No. 5,559,327. The mass spectrometer is mounted in a housing 10 with a prefilter 12, a primary mass filter 14, a collision cell 16, and a secondary mass filter 18 in serial alignment between an ion source 20 and an ion detection system 22. Bolts 13 and posts 15 are provided for mounting quadrapole segments. Ion source 20 and ion detector system 22 are provided at opposing ends of the filter and collision detector assembly. An ultra high vacuum pump 24 is centrally disposed in the housing to maintain a vacuum in a quadrapole element. Prefilter 12 is positioned in a first sealed region of the housing provided by the wall 30 and O-ring 32 which engages a cover (not shown) to provide the vacuum seal for a source pressure vacuum chambers. An ion lens 34 permits ions to pass from prefilter 12 to the primary mass filter 14 in a second sealed chamber of the housing provided by wall 36 and an o-ring 38 which mates with the cover (not shown) whereby a pressure on the order of 10xe2x88x926-10xe2x88x927 Torr can be maintained for an analyzer pressure vacuum chamber. Ion lens 40 is provided between the primary mass filter 14 and the collision cell 16, ion lens 42 is provided between collision cell 16 and secondary mass filter 18, and ion lens 44 is provided between secondary mass filter 18 and detector 22.
The function of a collision cell is to modify ions by either colliding them into fragments, or to react them with other molecules. In both cases, a parent ion is introduced into a higher-pressure region for a given time. The resulting fragment or product daughter ions are then exiting the cell. The collision or reaction energy can be varied by the parent ion""s initial velocity, the size of the collision gas, and the number of collisions encountered. The number of collisions is depended on the gas pressure and the reaction time. During the collision process, the charge of the parent ion will hang on to one fragment part, where as the remaining part will be neutral. These neutrals can pass through the next mass filter, and produce non-specific signals, reducing the sensitivity of the mass spectrometer. If a Parent ion hits a collision gas molecule, its flight path will be altered. For this reason most cell designs are built around multi-pole structures containing an ion focusing RF field.
A collision detector in accordance with this invention combines all these functions, while maintaining a continuous focusing of the ions throughout.
In accordance with the invention, an ion collision cell is provided which is lens-free by providing a high pressure collision region with pre- and post-evacuation sections. A continuous quadrature rod design reduces mechanical cost and simplifies electronic design. Ion node effects are minimized by eliminating small aperture ion lenses, and collision cell length permits lower pressure operation. By placing a curve in the collision region all neutrals (uncharged ions and ion fragments) will not be focused by the RF fields and will travel in straight lines and exiting the collision region of the cell. A square quadrapole, cross section allows a field free region in the center of the quadrapoles which minimizes node effects and allows a broad stable mass range for a given RF amplitude. An appropriate gap can be selected between adjacent rods to optimize the evacuation sections yet maintain ion stability. By adding a DC voltage to all rods, the parent ion entrance velocity can be easily adjusted over a wide range of energies.
In accordance with one embodiment of the invention, the lens-free ion collision cell comprises first and second pole segments mounted on a first support plate with the pole segments having pole surfaces arranged at approximately 90xc2x0 with respect to each other. Third and fourth pole pieces are mounted on a second support plate with the second pole segments having pole surfaces arranged at approximately 90xc2x0 with respect to each other. The first support plate and the second support plate are assembled together with a spacer for placing the first, second, third and fourth pole segments in juxtaposition with the pole surfaces arranged in a generally square cross section. A sealant is provided for vacuum sealing an intermediate length of the assembled pole segments, and a gas inlet is provided in the intermediate length for introducing a gas into the assembled pole segments. Evacuation ports are provided at opposing ends of the assembled pole segments.
The assembled pole segments can be linear in configuration or curved, such as 90xc2x0 or 180xc2x0. In operation the collision cell combines the functions of ion lenses or RF multipoles while allowing the gas from the collision cell to be pumped away and while maintaining a continuous focusing of the ions throughout the collision cell
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings.